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Photo: Infineon If you have an E-tag for the tollway, a micro-chipped pet or a latemodel car with an immobiliser key, then you’re already using radio frequency identification (RFID) technology. But this is only the start. Over the next few years RFID technology will start to replace bar code labelling systems. It might even be used to identify people! The implications are enormous. So what is RFID and how does it work? R adio Frequency Identification (RFID) has been around in one form or another since World War II. Although it has been used in niche industrial sectors for many years, the increasing desire for greater efficiencies in supply logistics have really pushed the development and use of this technology. An RFID system consists of a reader and transponders. Transponders (derived from the words “transmitter” and “responder”) are attached to the www.siliconchip.com.au items to be identified. They are often called “tags”. Just like a bar code, a transponder tag carries data about its host. When interrogated by a reader, it responds with that data over a radio frequency link. The transponder could be really simple, like those in clothing price tags, consisting of just an antenna and diode. When irradiated, the diode By PETER SMITH rectifies the incoming carrier and the frequency-doubled signal is radiated back to the reader which responds with an alarm if you try to leave the store without paying for the product. These days, the generic term “RFID” is used to describe an entire range of dedicated short-range communication (DSRC) systems. This article does not attempt to describe all RFID devices and technologies. Instead, we will focus exclusively on RFIDs used in identity July 2003  7 Fig.1: a basic RFID setup consists of a reader (or interrogator) and transponder. Low frequency systems rely on inductive coupling to provide transponder power. tagging and closely associated areas. Let’s begin by dividing the subject into two broad categories: active and passive transponders. Passive Transponders Passive transponders do not have an in-built power source; they are powered entirely from the magnetic/ electric field of the reader’s antenna. This energy is used to power on-board electronics as well as to transmit data back to the reader. Because of the close coupling requirements of the reader and programmer, reading distance is limited. It varies from a few centimetres to several metres, depending on the transmission frequency, power level and other factors that we’ll examine shortly. Passive tags come in a huge variety of shapes and sizes, depending on their application. They can be made to withstand extremely harsh environments. Without a battery to run flat, Active transponders are battery-powered and are generally designed for communication over greater distances than their passive counterparts. On-board power allows higher data rates and better noise immunity but active transponders are bigger, cost more and have a finite life. The E-tag for Sydney’s tollways is a good example of an active tag. Similar systems in Europe operate in the microwave spectrum, which implies very high data transfer rates. In fact, the European systems allow you to speed through the tollgates at up to 160km/h and are still able to successfully bill you for the trip! be limited to 20 characters, whereas tag memories can hold 512 bits (or lots more) of data. Importantly, the memory on some tags can be both read and written many times over, allowing “on-the-fly” data updates. Simpler tags that contain “WORM” (write once read many times) memory are also in use. Unlike bar coding schemes, “smart” tags include computational electronics, enabling encrypted, high security information exchange. This can be seen in action in the new contactless credit cards and “electronic purse” systems already in use throughout Europe. The invisible medium of radio also means that tags do not need to be “lineof-sight” to be read. With help from the on-board electronics, it also allows multiple tags (within reader range) to be read “simultaneously”. Imagine how all this might ultimately change your shopping experience. You could fill your trolley and wheel it directly out of the supermarket. Invisible readers at the exits would scan all of your items and charge your “smart” credit card while it’s still in your wallet (or purse)! No waiting at the checkouts – wouldn’t that be great? RFID advantages How passive systems work The fact that RFID is “contactless” is only part of its attraction. RFID tags carry much more data than bar codes. For example, a typical bar code might Passive tags usually consist of just a single IC and an antenna (coil). Currently, most passive tags operate below 100MHz and rely on the magnetic field they can last indefinitely. Active transponders Fig.2: block diagram of a typical low frequency reader. All high-level functions, such as data encryption/ decryption, collision detection and host communication are performed by the microcontroller. 8  Silicon Chip www.siliconchip.com.au Fig.3: a typical low frequency transponder. The transistor across the coil loads (or “damps”) the reader’s magnetic field to transmit data from memory. In most implementations, a single IC performs all of these functions. produced by the reader for both power and communication. The reader generates a carrier signal and this induces a voltage across the coil of the tag. This voltage is rectified and filtered to become the power supply for the IC. Some tags also divide down the carrier signal and use it as the clock for on-board logic, whereas others generate their own clock signal. Tag transmission Essentially, tag data transmission is achieved by switching a low resistance across the antenna coil. Loading the coil in this way causes a corresponding dip in the peak voltage across the reader’s coil. In other words, the change in voltage across the tag’s coil is reflected back to the reader’s coil. This is often referred to as “backscatter”. The serial data stream from ROM (and/or EEPROM/FRAM) memory does not directly drive the coil-loading switch. Instead, the switch is driven by a low-frequency clock source. This effectively superimposes a weaker “subcarrier” on the main carrier signal. Modulating this subcarrier performs actual data transmission. Without going into lengthy technical discussions, we can tell you that the modulation method may be ASK (amplitude shift keying), PSK (phase shift keying) or FSK (frequency shift keying). Serial data is typically Biphase, Manchester or Miller-encoded before transmission. stages is cleaned up with a Schmitt trigger and pumped into a digital logic block, where the original data is reconstructed through a demodulation and/or decoding process. Typically, all of these functions are performed by a single IC, supported by a few external (passive) components and perhaps an antenna power amplifier. Higher level functions, such as data encryption/decryption, collision detection and host interfacing are usually performed by a microcontroller, which is interfaced to the reader IC via a simple serial or parallel interface. Reader reception For two-way (read/write) systems, the reader must also be able to transmit data to the tag (to update the EEPROM/ FRAM). This is typically achieved by amplitude, pulse-width or pulse-position modulation of the carrier signal. In its simplest form, transmission to the tag is performed by switching the carrier signal on and off (100% amplitude modulation). A “gap detect” circuit in the tag serialises and demodulates the “gaps” and “no gaps” to reconstruct the original data. Once a complete data frame is received, it is checked for validity (using a CRC polynomial). If sufficient power is available, it is then committed to memory. In some systems, the carrier is not switched on and off but is modulated at a particular “depth” (about 10%). This makes more power available for In order to receive tag data transmissions, the reader’s antenna signal is first processed by analog front-end circuitry. Its main functions are to remove the carrier signal and then amplify the (much) smaller sub-carrier. The resultant signal from the envelope detection, filtering and amplifying A much larger-than-life computerrendered image of TI’s DST+ (Digital Signture Transponder Plus) module. These are embedded into vehicle keys to provide sophisticated fraud prevention information. The long ferrite rod coil and transponder IC are clearly visible. www.siliconchip.com.au Reader to tag transmission July 2003  9 tag use, extending range and enables smaller tag antennas to be used. Frequencies and antennas A collection of 14.35MHz tags and labels with TI’s “Tag-it” transponders hidden inside. Photo: Texas Instruments This is what’s inside the tags and labels. In bare format, the transponders are referred to as “inlays”. Photo: Texas Instruments The most common frequencies in use for passive RFID systems are 125kHz - 134.2kHz and 13.56MHz, with a few operating up in the 900MHz and 2.45GHz regions. The frequency of operation has a very big impact on system design, configuration and cost, and it’s all to do with “near” and “far” fields. Antennas radiating an electromagnetic field generate what is known as “near” and “far” field components. Most passive transponders rely on inductive coupling, so they utilise the “near” field component. The “near” field signal decays as the cube of distance (1/r3) from the antenna, whereas the “far” field signal decays as the square of the distance (1/r2) from the antenna. As you can see, the use of inductive coupling and “near” field severely limits the reading distance. However, this can be desirable, as it allows engineers to tightly control the radiating pattern and reach of the reader’s field. To borrow from our earlier supermarket example, it is possible to ensure that shoppers are only charged for what is in their trolley (and in their pockets!). Low frequencies and small tag sizes are two other important reasons for using the “near” field. For example, consider the size of conventional ¼-wave dipoles for 125kHz (or even 14.35MHz) that would be needed for “far” field communication. These would need to be 600m and 5.23m long, respectively; much too big for integration into a pea-sized transponder or credit card! For inductive coupling, the antenna (we use the term loosely) must be resonant at the chosen carrier frequency. This is achieved by adding some parallel capacitance (for the transponder) or series capacitance (for the reader) to a known value of antenna inductance. Size does matter Reader size varies according to application. Miniature units with built-in antennas are available, whereas store-front models need walk-through antenna loops. Here are two semi-portable (14.35MHz) readers from TI. As indicated in the foreground, these models are designed for ID card use. Photo: Texas Inst. 10  Silicon Chip Reader and transponder antenna size is a critical factor in “near” field systems. As the tag size is generally fixed (in credit card form factor, for example), the reader side becomes the variable. Many manufacturers quote a “rule of thumb” reading distance roughly equivalent to the diameter of www.siliconchip.com.au the reader’s antenna. Identification Numbers. However, it’s important to 240,000 books and 60,000 CDs note that factors such as antenna and DVDs in Vienna’s new main orientation, radiated power and library have been equipped with environmental conditions all RFID transponders. Self-service terhave significant effects on reading minals in the library make checkout distance. completely painless. For 125/134.2kHz systems, the Mobil has teamed up with Texas antennas (OK, the coils!) are conInstruments to create a hybrid acstructed with many turns of wire, tive & passive transponder system often wound on ferrite cores to for petrol purchase. Based on TI’s reduce size. Transponder coils can TIRIS system, it enables thousands be as small as a cm or two, making of motorists in the US to fill up them ideal for animal tagging (imwithout the need for cash or even plants) and car security systems. a card. Transponders in both the Data transfer speed is typically car and the driver’s key ring make between 2 - 10kb/s. a positive ID as soon as the vehicle pulls up to the pump. Now all they By contrast, 14.35MHz tranneed is a robot to fill the tank… sponder antennas require less than Close up of a Tag-it inlay. The tiny black dot 10 turns (the readers may have only is the transponder IC, with the antenna coil occupying most of the remaining space. These Where to from here? one turn), which is easily printed as a foil pattern for tag inlays or etched inlays are small and highly flexible and can Despite all this activity, there are directly onto PC boards. This fre- be attached to almost anything. Photo: Texas still some wrinkles to be ironed out Instruments quency is widely used for credit before you’ll see RFID in use in cards, identity tags, anti-theft layour local supermarket. The lack bels and bar code replacements. Data of international RF standards (bands The company now has the ability to transfer speed at this frequency is up track the tagged garments even after and power levels) is frustrating develto 100kb/s. purchase, which is proving to be a opment. In addition, the cost per tag is still prohibitive for use on low-cost somewhat controversial ability. UHF/microwave systems products. The London public transportation Passive systems that operate in the The bean counters tell us that tags system is installing a smart ticketing 900MHz and 2.45GHz regions are also system that uses contactless smart must be priced at less than 1% of the in use. The considerably shorter wave- cards. This is reputedly the largest products they’re attached to. Recent length of these frequencies allows the project of its kind to date, with 80,000 reports indicate prices as low as 10c use of dipole antennas (usually 1/8staff already issued with Philips apiece but that’s still too expensive wave) and the “far” field emissions for the frozen peas and baked beans. MIFARE cards. of the reader. On-going research into organic Michelin engineers have develReader range is considerable longer oped RFID transponders that can be semiconductors might prove to be the (>3 metres) than for lower frequency embedded into their tires, to store in- ultimate answer. Using this emerging systems. However, microwave fre- formation such as maximum inflation technology, it may soon be possible to quencies are highly directional and pressure, tire size, etc. It also allows “print” transducers just as we currentSC readily absorbed by organic tissue, tyres to be associated with Vehicle ly print barcode labels! which makes them unsuitable for many applications. High frequency tags also require precision manufacturing and more expensive electronics than their lower-frequency counterparts but they can support data rates of 2Mb/s or more. RFID in the news High profile manufacturers and retailers like Proctor & Gamble, Gillette, Wal-Mart and Tesco are currently trialing RFID technology. They’re employing “smart” shelves that keep track of stock using transponder tags. When stock levels drop too low, the shelves automatically notify staff. Benetton have embraced the technology, sewing Philips I.CODE tags into thousands of their retail products. www.siliconchip.com.au 125/134.2kHz transponder modules can be manufactured in almost any shape and size, as demonstrated by this collection. Photo: Texas Instruments July 2003  11